<?php
namespace app\modules\backend\utils;

class Crypt {
	public static function encode(string $str, string $key, int $expire = 0) {
		return self::code($str, $key, false, $expire);
	}

	public static function decode(string $str, string $key, int $expire = 0) {
		return self::code($str, $key, true, $expire);
	}

	// 字符串解密加密
	private static function code(string $string, string $key, bool $mode, int $expire) {
		$ckey_length = 20; // 随机密钥长度 取值 0-32;
		                   // 加入随机密钥，可以令密文无任何规律，即便是原文和密钥完全相同，加密结果也会每次不同，增大破解难度。
		                   // 取值越大，密文变动规律越大，密文变化 = 16 的 $ckey_length 次方
		                   // 当此值为 0 时，则不产生随机密钥

		$key = md5($key);
		$keya = md5(substr($key, 0, 16));
		$keyb = md5(substr($key, 16, 16));
		$keyc = $ckey_length ? ($mode ? substr($string, 0, $ckey_length) : substr(base64_encode(md5(microtime(), true)), 0, $ckey_length)) : '';

		$cryptkey = $keya . md5($keya . $keyc);
		$key_length = strlen($cryptkey);

		$string = $mode ? base64_decode(substr($string, $ckey_length)) : sprintf('%010d', $expire ? $expire + time() : 0) . substr(md5($string . $keyb), 0, 16) . $string;
		$string_length = strlen($string);

		$result = '';
		$box = range(0, 255);

		$rndkey = array();
		for($i = 0; $i <= 255; $i ++) {
			$rndkey[$i] = ord($cryptkey[$i % $key_length]);
		}

		for($j = $i = 0; $i < 256; $i ++) {
			$j = ($j + $box[$i] + $rndkey[$i]) % 256;
			$tmp = $box[$i];
			$box[$i] = $box[$j];
			$box[$j] = $tmp;
		}

		for($a = $j = $i = 0; $i < $string_length; $i ++) {
			$a = ($a + 1) % 256;
			$j = ($j + $box[$a]) % 256;
			$tmp = $box[$a];
			$box[$a] = $box[$j];
			$box[$j] = $tmp;
			$result .= chr(ord($string[$i]) ^ ($box[($box[$a] + $box[$j]) % 256]));
		}

		if($mode) {
			$expire = substr($result, 0, 10);
			if(preg_match('/^\d{10}/', $expire) && ($expire == 0 || $expire - time() > 0) && substr($result, 10, 16) == substr(md5(substr($result, 26) . $keyb), 0, 16)) {
				return substr($result, 26);
			} else {
				return '';
			}
		} else {
			return $keyc . base64_encode($result);
		}
	}
}
